There are generally three main classes of gravimeters: (a) laser or atom interferometers using timed measurements, (b) cryogenic superconducting levitated masses, and (c) spring-type gravimeters.
Laser interferometers have been implemented commonly for precision metrology across many scales and allow absolute gravimetry measurements with 1 to 10 μGal accuracies. Typically, laser interferometers involve timed and multiple-sampled measurements with calibrated or stabilized lasers, including locked to atomic clocks, to measure the free-fall of a reflecting body. Recent advances, for example, have used cold atom interferometry to determine the gravitational redshift to an accuracy of 7×10−9, improved precision of the gravitational constant to 1×10−4, or the gravity to a sensitivity of 100 ng per shot. With the interferometric or timed measurements, however, significant isolation from the environment—be it laser stabilization or cooling—is often required, which might hinder portability or rugged field deployment realizations.
Superconducting gravimeters typically have low thermodynamical noise and low-drift, which can be due to the inherent stability of persistent currents in the superconductor, stability of the mechanical proof mass (e.g., a few grams), and insensitivity to ambient perturbations. Superconducting gravimeters, however, typically operate at cryogenic temperatures at ˜4.2K or lower the even in a closed-cycle cryostat requires ˜1 kW power for helium liquefaction, bringing challenges outside the laboratory environment.
The third class of gravimeters provides the spring-type approach for relative inertial force measurements. This approach is generally the most deployed. Prior work in the bulk involved simply an inclined spring to a cantilever beam (e.g., 10 cm spring) that gives a ˜100 nm displacement for a ˜10 ng relative gravity difference. This displacement can be sensed optically. The ensuing linearity about the zero-displacement point can provide a large measurement range; the use of quartz beams can alleviate concerns such as, e.g., hysteresis and fatigue in the sensor. This baseline design has been continuously modified and updated by, for example, Scintrex and sister company Micro-g La Coste, encompassing applications such as, e.g., mapping the deep ocean seafloor morphologies. In one particular implementation, the recent gPhone can achieve, for example, 100 nGal resolution, 1 μGal precision with a system noise of 3 μGal/Hz1/2, 7 Gal range and 1.5 mGal/month drift. This bulk unit can also include a rubidium clock to synchronize the global positioning system. A compact chip-scale gravimeter, however, till date only has a few initial recent suggestions, involving, for example, gravimeters with capacitative readout.